Ataxia-telangiectasia (A-T) is a complex genetic disorder, characterized by cancer predisposition, immune deficiencies, and other defects. Cells from these patients are radiosensitive and have defective cell cycle checkpoint responses, which places the function of the gene at a critical position in the response to DNA damage. In addition, evidence has accumulated that A-T cells exhibit a mis-repair phenotype affecting both non-homologous and homologous recombination, which implies that at least two genetically distinct repair mechanisms (protein complexes) are affected by a single gene product. It is therefore hypothesized that the ATM product is responsible for the processing of double-strand breaks of any type, before it is decided whether repair will follow the homologous or non-homologous recombination pathway. Our working model is that DNA damage is 'sensed' as an early event and rapid response proteins are recruited to sites of damage, which perhaps eliminates DNA fragmentation (with a half-life of 30 minutes). From the initial event, multiple signal pathways are initiated, one of which presumably involves ATM. The main theme of this proposal is that mis-repair or abnormal processing of broken DNA ends is the direct consequence of lacking the ATM protein and that this occurs independently of the defects in cell cycle checkpoints. First we will characterize the extent and consequences of mis-repair in chromosomally integrated plasmid substrates at both, the plasmid and the chromosomal level, as well as in extra-chromosomal systems. We will next determine, by a genetic approach, whether the cell cycle checkpoint function of ATM can be separated from its role in DNA repair. These studies will also ask to which extent the A-T phenotype, i.e., cell cycle checkpoint defects, genomic instability and mis-repair, can be attributed to p53-dependent pathways, as this will be crucial for understanding the cancer susceptibility in A-T. Finally, as the Nijmegan- Breakage syndrome (NBS) has an A-T like phenotype we will investigate whether a similar DNA repair-defect can be defined in NBS cells. Functional analysis will address the relationship between the NBS- containing protein complex and ATM in a DNA damage response pathway.